Stop thinking about it. If you have a indoor Foscam, and you are spending the time worrying about signal strength, and you have invested your time, money, and wishes on the cameras, just get these antennae for EVERY one of your cams. Here's why:
Unless the Foscam is in the same room as the router, an IP cam puts a lot of strain on your WiFi. Unlike web pages or email, a cam streams video, video is high bandwidth, and if you're storing your video on a PC (usin Blue Iris software, for example) the cam is streaming 24x7. Add a few more cams, and that's a lot of bandwidth.
As your wifi channel becomes slightly congested, and your signal strength is not ideal, the cameras start dropping data packets, and trying to re-send, and then conflicting with the other cameras which all want to send their video NOW! Stronger signal avoids the lost packets and the resends. Use these, or even better, run Ethernet cable. You won't believe how much better the cameras work on Ethernet.
By running all upgraded antennae, you will get better framerates from all cams, more reliable connections, the ability to have more cams on the same wifi network, fewer crashes, fewer bugs, faster PTZ response, etc. That's true even if your system works just fine with the shorties.
Here's the downside, which is manageable: these antennas aren't magic. They don't make your signal stronger, they just change the aim of the signal. With the shorter stock 3dbi pole antenna, picture a big donut around the tip of the antenna. That is where your signal goes. Now, with this longer 9dbi pole antenna, picture a flat pancake around the tip of the antenna.
That is where your signal goes with this unit. That means you have to aim the antenna in such a way that the wifi AP is on a plane perpendicular to the antenna. If your Foscam is one floor above your WiFi, this antenna will make your signal WORSE if you keep the antenna straight up, and don't "aim the pancake".